overheating race car - stumped!

[cbuczesk]

I have a 1972 240Z that I race in ITS. I built a new engine and used it for a race weekend last month without any issues except for a miss that would kick in around 6400-6600 RPMs. After the race I rebuilt the carbs and took the car for a drive. The miss was still there but everything else was fine including water temp.

I went out for qualifying Saturday morning and had the annoying miss but water temp was good. I swapped out the MSD 6 digital with the MSD 6AL that I used last year in an attempt to fix the miss. The race was Saturday afternoon and the miss was a little better but the water temp got up to almost 260 on the 6th lap so I came in. There was so much pressure in the system that the overflow hose blew out of the catch bottle! Once the car cooled I removed the thermostat.

It was warmer Sunday and on the 3rd lap of qualifying the temp hit 240 and I came in. This time I put on a known good water pump and a new radiator cap. I ran the engine with the cap off and I could see coolant flowing in the radiator. The front carb was running lean so we adjusted it. In the race the car hit 240 on lap 3 and I backed it down. I was able to complete the race by racing 2 laps and then coasting down the front straight and short shifting a lap and repeating that process.

I'd appreciate any ideas!

Chuck

[Pomorza]

Chuck

A few questions. Is the miss still there? Did you break in your new engine properly? What shape is the radiator (and cap) in? I would start with the obvious, pull the plugs and see what's going on there. If you have a miss that's still there you clearly have an issue of some sort.

Just a few suggestions

Jan

[madkaw]

You swapped out ignitions-did this change any of your timing parameters?

Have you done a radiator pressure check to insure no HG defect?

Do you run with a thermostat? If so, what kind?

[Zed Head]

I would start with the obvious, pull the plugs and see what's going on there.

Instead of obvious, scary might be more appropriate. Headgasket,maybe? New engine, cylinder misfire and overheating.

[cbuczesk]

The miss is still there but it doesn't happen until 6700RPMs. I'm thinking that the problem is with the distributor and that the digital MSD is more sensitive than the analog that's in it now. I'll post a link to my in-car camera tonight so you can hear the miss. You might be able to read the temp gauge too.

I broke in the engine properly. 20 over pistons, new bearings, etc. The engine has to remain mostly stock for ITS.

Pulled the plugs before yesterday's race and 1-3 were lean. 4-6 were fine. I will pull them when I get home today.

The radiator and cap were new last year. It's a 3-row from Champion. I used it last year and it kept the engine plenty cool.

Chuck

[zKars]

Ideas:

-Losing coolant? But not to the overflow? Do a leak down test for sure. Is one cylinder disposing of the coolant due to a HG failure and causing the mis-fire? Plugs might be a clue if one looks different than the rest.

-Bad hoses, ie collapsed lower hose on the suction side.

-Front cover damage in the impeller area causing poor pump performance. Check for chips and cavitation caused pock marks in the impeller housing area.

-If using a thermostat, take it out and see if problem changes/improves.

[cbuczesk]

I didn't check the timing after swapping the MSD. I assumed that timing wouldn't change but I plan to check that tonight.

I haven't done a pressure test but all the coolant hoses were pressurized when the engine was warm.

I started Saturday with aa themostat but removed it for Sunday.

I'll pull the plugs tonight and do a cold and warm compression test. I have a new leakdown tester that I can learn to use.

Thanks for all the ideas guys!

Chuck

[cbuczesk]

It looks like I found the problem. Here are my compression numbers from a cold engine: 157, 163, 175, 175, 165 183

I never saw any smoke from the exhaust and it didn't smell like coolant. If it burned the coolant it didn't burn much of it because there was very little missing from the radiator.

Now I have to figure out what caused the head gasket fail. I'm assuming that's what happened.

Chuck

[240260280z]

High RPM spark consistency is also affected by the meshing of the distributor drive spindle with the crank gear. There are combinations of steel and brass that are supposed to be better than stock.... I read this it in one of the datsun racing or datsun engine rebuild books.

Maybe sunken brass seats or a valve adjust is the reason for the variance in compression.... did you baseline it after the rebuild?

Edited July 16, 2013 by Blue

[Zed Head]

It looks like I found the problem. Here are my compression numbers from a cold engine: 157, 163, 175, 175, 165 183

I only suggested the head gasket because some of the signs fit, but the signs also fit some of the other suggestions, like a collapsing hose, along with an ignition problem, for example. Those pressure numbers sure don't look good though.

You have so many cars, and you're building your own engines, so I'm sure you know how to get good test numbers. Since it's so new and has few miles, I wonder if re-torquing the head bolts could save it? Not likely, but a re-torque would tell you if any of the bolts were loose, before you removed them to work on the gasket. If you find loose bolts, at least you'll have a possibility for the cause.

[madkaw]

I had a bad HG and my numbers looked better then that, so not sure that tells all.

The tester kit I suggested from Autozone(freebie borrowed tool) plugs into the radiator instead of the cap. You run the engine and see if the pressure rises above cap pressure. If it does, start pulling plug wires one at a time to try and diffuse the pressure. When the pressure stops rising, you found where your problem is. My symptoms were very subtle, never overheated, but I wasn't racing either. It just didn't run right, like a miss.

That was a felpro gasket I believe, and it was giving out around #3. I'm running a Nismo gasket now.

I also haven't read where you checked valve lash.

[Wade Nelson]

At 6500 RPM's the thing that usually goes wrong is a valve starts floating. I'd pull your springs and check them all; you may have a weak one.

As for overheating, sorry, we don't provide magic bullets here on ClassicZ but there are any number of overheating threads you can read and learn from, including this one:

http://www.classiczcars.com/forums/help-me/49737-temperature.html

I believe that once one understands the THEORY of overheating finding the mechanical cause is fairly straighforward.